1. Field of the Invention
This invention relates to integrated circuits and more particularly utilization of fuses therein.
2. Description of the Related Art
Polysilicon agglomeration fuses are often used in microprocessors to implement small PROM arrays which are used to store various configuration information. Typically, the small PROM arrays are programmed as part of the testing process during manufacturing. A polysilicon fuse is simply a short piece of polysilicon that is used as a fuse element. To xe2x80x9cprogramxe2x80x9d the fuse element a large current is passed through the fuse element, which then heats up and changes the resistance of the fuse element permanently. The resistance of a fuse is measured using a xe2x80x9csensingxe2x80x9d circuit to determine if the fuse has been programmed or not. A programmed fuse will be read as one logic sense (e.g., a xe2x80x9c1xe2x80x9d) while an unprogrammed fuse will be read as the opposite sense (e.g. a xe2x80x9c0xe2x80x9d). In this way information needed to configure the internal operation of a processor can be programmed into the fuse-based PROM array and stored there permanently.
Since xe2x80x9creadingxe2x80x9d a fuse element involves measuring the resistance of a piece of polysilicon, the circuitry employed to read the fuse elements usually involves relatively complex and, sometimes, area intensive analog circuitry such as sense amplifiers, current references, voltage references, current mirrors and the like.
Using such analog circuitry makes testing the fuse sensing circuits difficult without adding often complex additional support circuitry. It is desirable to simplify the circuitry for sensing and testing as much as possible in order to streamline the design process and reduce the silicon area needed for the fuse sensing circuitry.
Accordingly, the invention provides a circuit for sensing fuses that is extremely simple, can be implemented in a relatively small area and lends itself well to testability.
In one embodiment, an integrated circuit includes at least one fuse cell circuit, which includes a first fuse and a first fuse sense circuit coupled to sense a programming state of the first fuse and supply an indication thereof. A sense control circuit includes a plurality of reference fuses and a second fuse sense circuit coupled to the reference fuses. The sense control circuit supplies a sense control signal to the fuse cell circuit to cause the fuse cell circuit to sense the programming state of the first fuse when the sense control signal is asserted. The sense control signal is asserted for a time period determined , at least in part, by a resistance value of the reference fuses. The integrated circuit may also include a resistance varying circuit coupled to vary a resistance value of the reference fuses according to one or more control signals.
In another embodiment a method is provided for determining a programming state of one or more fuses. The method includes receiving a read fuse request in a sense control circuit and generating an asserted sense control signal in response to the read fuse request. A current is caused to flow through a reference resistance in response to the asserted sense control signal. The sense control signal then deasserts according to a voltage level present at one end of the reference resistance when the current flows through the reference resistance. The method may further include supplying the sense control signal to at least one fuse cell including a fuse and a sensing circuit, to cause a sense current to flow the fuse. A voltage generated at a first end of the fuse is sensed to determine the programming state of the fuse. The method may further include varying the reference resistance.